A four-man Earth crew land on Mars to look for the previous two expeditions, which disappeared without a trace. The crew approaches a city and their leader, Captain Williams, tells his men to hide their guns. The men, Wilson, Rollins, and Young obey. A Martian woman, Mrs. Ttt, irritably greets them and demands to know why they're taking up her time. When they wonder how she can speak English, she explains that she's telepathic. They try to impress upon her how significant their visit is, but she isn't interested and talks about delusions. She tells them to go down the road and talk to Mr. Ddd. When they try to get her attention, she gives them a note and tells them to talk to Mr. Aaa at the next farm over, and then slams the door in their face.
The crew goes to the next farm, but Mr. Aaa is more concerned with Ttt trying to annoy him. He grabs a gun but briefly tells them he's going to go after Ttt, and he could care less about them. He says that the Earthmen aren't his business and that Mr. Iii is the person they need to talk to. Mr. Aaa briefly gives them directions and goes on his way.
The Earthmen have no choice but to go to Mr. Iii. He's interested in seeing them but only wants to let Captain Williams in the house. When Williams asks if the others can come inside, Iii doesn't see the point but doesn't see the harm, either. He uses telepathy to read Williams' mind and seems impressed that he's from Earth, and then asks him to sign some forms. Williams has no choice and wonders if the others should sign. Iii jokingly agrees and then tells them that Mr. Xxx will deal with them next. He gives them a key to a door in the house, and tells them to go inside and wait for Xxx. Williams waits, expecting to receive a hero's welcome, and Iii shakes his hand.
The exploration team goes to the designated room and finds it filled with Martians. They hesitate at first, but when Williams explains who they are, the Martians a cheer and throw a party. However, the Martians begin introducing them as fellow Earthmen. However, each one spouts a different and contradictory description of the Earth that they are from. When Wilson tries to unlock the door they came through, he discovers that they've been locked inside. Williams is the first to realize what happens: they've been locked up in a psychiatric ward. The crew realizes that delusions of being from Earth are commonplace on Mars. Each person they encountered was so used to it, they simply sent them on until they reached the mental hospital. Since Martians are telepaths, even mind-reading doesn't prove they are insane, since they are able to create manifestations of their delusions.
Mr. Xxx, a doctor, finally arrives. He only addresses Williams, who tries to explain. Xxx explains that since he is delusional, the rest of his crew is projected hallucinations which don't really exist, which is why Mr. Iii all but ignored them. The captain insists that Xxx touch his officers, and Xxx concedes that it's a very convincing telepath projection. When Williams asks to be cured, Xxx admits that there is no way for sure to tell if Williams will ever be cured. The frustrated crew finally decides to show Mr. Xxx their lander.
At the landing site, Mr. Xxx walks around the lander and then enters it. When he emerges, he congratulates Williams on being the most powerful delusional telepath on the planet. Mr. Xxx informs the despairing Williams that he signed papers allowing the psychiatrist to do whatever is necessary to preserve order. Warning that Williams' delusions could be contagious, Mr. Xxx takes out a gun and shoots the captain dead. He waits for the three crewmen, Williams' delusions, to disappear. When they don't, he shoots them as well. However, Mr. Xxx is shocked to see that the corpses and the lander remain, despite the fact that Williams is dead. He can only conclude that he has caught Williams' telepathic insanity. With no cure possible, he shoots himself.
As the corpses and the lander bleach beneath the sun, the lander's mothership tries to make contact with their missing crew.